
Although Tim Tebow isn't the exact same QB as Manziel, he is surely as close to Manziel that you can find as a back-up. You can use the same playbook with Tebow as with Manziel. Just use a few more running plays and a few less passing plays.
Reason 2 - Manziel is a QB that can extend plays and pick up long runs. He's going to get hit a lot and he's going to get hit hard. Add to that he is a smaller guy (compared to other NFL players), Manziel could get hurt easily. You're going to want a QB that can keep you in the playoff picture (if that's where you are) and that you won't have to change your entire offensive scheme.
Reason 3 - How about actually playing Tebow for a fourth of the game or a third of the game. This way, not only is Tim Tebow your "fire insurance" if Maziel misses any time, but he can also be a "fire prevention." You can keep Maziel's snap count down. Keep his hits down. This will keep Manziel from being hurt in the first place.

Whether you think Tebow is a "good quarterback" or not, we do know that he went 9-3 with the Broncos starter including a playoff win in a system set up for him. Tebow, at the least, can be a productive back-up QB in a system that is conducive to his style of play.